The present invention relates to photomasks or reticles used in the fabrication of semiconductor devices. More particularly, the present invention relates to phase shifting photomasks and processes for designing them.
Advances in capacity in semiconductor chips have generally been the result of decreases in the size of the features on the chip. The lateral dimensions of features are generally defined by photolithographic techniques in which a detailed pattern is transferred to a photoresist by shining light through a mask or reticle.
In recent years, phase shifting masks have been developed to improve photolithographic processes. Phase shifting masks increase image contrast and resolution without reducing wave length or increasing numerical aperture. These masks also improve depth of focus and process latitude for a given feature size.
With phase shift photolithography, the interference of light rays is used to overcome the problems of diffraction and improve the resolution and depth of optical images projected onto a target. With this technology, the phase of the exposure light at the target is controlled such that adjacent bright areas are preferably formed 180.degree. out of phase with each other. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the target.
In general, a phase shifting mask is constructed with a repetitive pattern formed of three layers of material. An opaque layer provides areas that allow no light transmission. A first transparent layer provides areas which allow close to 100% of the light to pass through. A transparent phase shifting layer provides areas which allow close to 100% of the light to pass through but phase shifted 180 degrees from the light passing through the first transparent layer. The first transparent layer and the phase shifting layer are positioned such that light rays diffracted through each area are cancelled out in a darkened area between them. This creates a pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer on the semiconductor wafer. Another method of constructing a phase shifting mask utilizes a semitransparent layer to cause the phase shift.
One process for fabricating phase shifting masks includes forming an opaque layer on a major surface of a transparent substrate, patterning the opaque layer to expose portions of the underlying transparent substrate, forming a phase shifting mask layer to expose the portions of the underlying transparent substrate, phase-etching the exposed portions of the transparent substrate until a 180.degree. phase shift is accomplished. Other processes of fabricating phase shifting masks include those in which a transparent film is formed over a portion of a mask to create a phase shift as well as the etching of rim shifting channels into the mask substrate.
With their increasing complexity, photomasks have become more difficult to design and fabricate over the years. Accordingly, its is desirable to fabricate photomasks which have the longest useful life possible. Additionally, it is advantageous to design photomasks which have the widest application possible.
Accordingly, it would be a significant advancement in the art to provide a photomask and method of fabrication wherein the photomask could be used to print features with different sizes or different configurations from the same mask. Such a photomask is disclosed and claimed herein.